This application relates in general to continuous mixers, and more particularly to a system and method of controlling a continuous mixer having a moving surface discharge device.
Continuous mixers for mixing plastic materials which include a mixing enclosure with interspaced entrance and exit openings and a mixing means are well known. In general, the mixing time of the material in the enclosure is dependent on the volume of material in the enclosure, which in turn is dependent in part on the rate at which material in the enclosure is moved out through the exit opening. By varying the size of the exit opening, the pressure required to push the material through the opening and the pressure on the material being mixed internally in the enclosure may be controlled.
In some situations, the restraint exercised by the exit opening is not satisfactory to exert the desired pressure on the material because of material characteristics of the mixing. Attempts to obtain a smoother action by use of walls having polished or lubricated surfaces have not been completely satisfactory either.
A smoother action is obtainable where the restraint to the flow of material discharged from the continuous mixer is provided by a moving surface discharge device in which one or more surfaces contact the material and move in the direction of the flow of material at controllable speeds. Movement at a speed initially slower than that of the flow when unimpeded restrains the flow and effects a back pressure on the flow. Then, by adjusting the speed so that the rate of feed into and from the mixer is substantially the same, this back pressure remains and is reflected back on the material being mixed in the mixer while moving through the mixer. Adjustment of the moving speed of the surface or surfaces adjusts the internal mixer pressure to give and maintain the pressure desired for the mixing operation involved. The speed can be made automatically responsive to the temperature of the mixing or discharging material, temperature being a function of the pressure on the material being mixed.
A screw-type extruder or a gear pump may be used to provide such a moving surface, provided that the rotative speed of the extruder screw, or pump gears, can be controlled. Continuous mixers utilizing a screw-type extruder and a gear pump are taught by U.S. Pat. No. 4,310,251 issued to Scharer et al.
In controlling the operation of a continuous mixer with a discharge device such as an extruder screw or gear pump, it is important that the operation of the starve fed continuous mixing device be synchronized with its closely coupled discharge device in order to achieve stable operation. It is also desirable to provide for the startup and shutdown of both pieces of equipment.
In U.S. Pat. No. 4,452,750 issued to Handwerk et al, an in-line melter/mixer-gear pump system is provided for the processing of synthetic, thermoplastic materials. The system employs the pressure between the melter/mixer and the gear pump as the controlling parameter which affects, in a proportional relationship, the speed of the gear pump, the energy transmitted to and the consequent temperature of the materials passing through said melter/mixer.
Such a control method has not proven to be of practical use due to the difficulty of developing relationships between the gear pump inlet pressure and the mixer energy input level. In fact, the relationship of pump speed to mixer power taught by Handwerk et al may be impossible. With the gear pump being a volumetric displacement device, it has a relatively constant output per revolution. It is, therefore, not possible to maintain an output rate of 900 lbs/hr over a gear pump speed range of 75 to 93 RPM. Either the material will back up in the mixer at the low end of the speed range (meaning the discharge rate from the system is no longer 900 lbs/hr) or the gear pump is running starved (incompletely filled) at the high end of the speed range, meaning that control of the mixer is no longer strictly due to the gear pump, but is now partially due to drag in the space between the mixer and the gear pump.
The Handwerk et al method of control also has the drawback that the pressure existing at mixer discharge is a derived variable, i.e. for the desired level of work input into the material being mixed, a certain pressure will exist. However, this pressure must be derived empirically by trial and error until the appropriate pressure for the desired level of work input in the mixer is achieved. As a result, such a system is difficult to use when starting up the machine.
It is therefore a principal object of the present invention to provide a system and method for controlling a mixer with a moving surface discharge device which can reliably control the speed of the discharge device and thereby control the energy transmitted to and the consequent temperature of materials passing through the melter/mixer.
A further object of the present invention is to provide a system and method for controlling a continuous mixer with a moving surface discharge device which does not require the use of a special measurement apparatus.
Still another object of the present invention is to provide a system and method for controlling a continuous mixer with a moving surface discharge device which can easily be adjusted to exert different pressures on the material being mixed.
Yet another object of the present invention is to provide a system and method for controlling a continuous mixer with a moving surface discharge device that operates as well during startup as during operation at the desired capacity.